1. Field of the Invention
The present invention relates to improvements in low-pass, RC active filter network apparatus. More particularly it relates to improvements which are of special utility in applications of such apparatus for the reduction of noise or other higher frequency signal components in a signal channel carrying pulse amplitude modulation (PAM) signals.
2. Description of the Prior Art
It has heretofore been considered that the application of low-pass RC active filter networks for noise rejection in conjunction with a PAM channel required as active filter network gain approximately equal to the reciprocal of the duty cycle of the PAM signal. This level of gain was only achieved by the cascade connection of two or more four-pole active filter network stages.
The use of a low-pass RC active filter network having at its inputs an analog circuit type multiplexing switch which operates in synchronism with PAM pulses is known in the construction of transmission equipment for telephonic communications. However, prior to the present invention, the inherent degradation of in-band frequency response was merely offset by the cascade connection of more, or higher gain, active device stages.
In addition to the aforementioned considerations given to gain requirements and band pass characteristics, there is interest in improvement of the out-band rejection capabilities of such filter network apparatus.